Best Compression technique

I am working on a sample backup code, where i read the files per 7200 bytes and send it to server. Before sending to server, i compress each 7200 bytes using zlib compression algorithm using dictionary max length of 1.5 MB . I find zlib is slow.

Can anyone recommend me a compression technique to satisfy my above requirements.
I saw lz4 compression, but it does not support dictionary in it.